Motor vehicle seat belt retractors are usually one of three types, ALR, ELR, or dual-mode. An ALR includes a self-locking mechanism operating like a ratchet while a seat belt tongue is engaged with a seat belt buckle. The seat belt webbing is automatically tightened, while the ALR prevents extraction of the available belt webbing. The ALR racheting operation is cancelled when the webbing is fully retracted. An ELR includes a locking mechanism that is activated by vehicle acceleration (vehicle sensitive), webbing acceleration (webbing sensitive), or other crash sensing means to block webbing extension. Absent an activation of the ALR, seat belt webbing can be pulled out to lengthen the available seat belt webbing. Both an ALR and an ELR are configured to restrain a passenger during a frontal impact or vehicle rollover. A dual-mode retractor combines an ELR and an ALR, and a vehicle occupant chooses the desired mode. Typically, a dual-mode retractor is switched to the ALR mode by completely or nearly completely pulling out the seat belt webbing. The ELR can be converted to an ALR at the option of a vehicle occupant. While the ELR mode provides a comfortable adjustment for an occupant seated on a vehicle seat, infant carriers may be more safely secured in the ALR mode. An ALR has the advantage that is very easily used with child restraint systems, since the belt can be fastened and cinched down to secure the child restraint. However, an ELR is more acceptable for ordinary use since it provides enhanced comfort and convenience for the vehicle occupant. Dual-mode systems accommodate use with both child restraint systems and normal operation.
In a safety system that comprises at least one further restraint device, such as an airbag or a load limiter for the seat belt webbing, the currently selected mode of a dual-mode retractor constitutes a valuable piece of information for the operation of the additional restraint device. For example, it is generally not preferred to activate a frontal impact airbag for a seating position being used for securing a child restraint device.
Accordingly, some vehicle manufacturers are looking to verify the currently selected mode, i.e. ALR or ELR, so they can use this information to provide as an input to an electronic control unit (ECU) for controlling certain restraint system functions. Some current retractors use microswitches or simple contact switches as part of a detection circuit to comply with the seat belt use indicator requirement in FMVSS 408. These are simple switches and may not provide the proper information for use in a restraint system control algorithm included in an ECU.